Fix filter controls break after changing audio channels

When the number of audio channels is changed, the producer changes.
But the filter controller does not know about it and continues to
use the old one.

src/mainwindow.cpp

@@ -534,6 +534,7 @@ void MainWindow::setupAndConnectDocks()
             m_filterController->motionTrackerModel(), &MotionTrackerModel::removeFromService);
     connect(m_timelineDock->model(), &MultitrackModel::removing,
             m_filterController->motionTrackerModel(), &MotionTrackerModel::removeFromService);
+    connect(this, SIGNAL(audioChannelsChanged()), m_filterController, SLOT(setProducer()));
 
     m_markersDock = new MarkersDock(this);
     m_markersDock->hide();